With interest in improved energy efficiency and the use of alternative energy sources, it has been recognized that vehicles, especially those that make frequent stops and starts such as delivery vehicles, could be made more efficient if the energy normally lost in decelerating or braking the vehicle could be somehow collected, stored and reused to accelerate the vehicle. Hydraulic accumulators can be used to store such energy.
A hydraulic accumulator is a device that stores potential energy in the form of a compressed gas or spring, or by a raised weight to be used to exert a force against a relatively incompressible fluid. It is often used to store fluid under high pressure or to absorb excessive pressure increases. The increase in pressure within an accumulator is directly related to the amount of stored energy (charge) available in the accumulator at any given instant. To reliably utilize the stored energy within an accumulator, and optimize a hybrid type system, a sensor is required to accurately measure and report the potential energy stored within an accumulator.
For example, accurately knowing the amount of charge in an accumulator allows control software to “project” the contribution the main engine will have to provide in order to maintain the proper power reserves during operation. Accurate charge data also allows for full utilization of the accumulator charging range. Additionally, having an accurate sensor with the ability to monitor an accumulator's charge state allows a system to quickly detect hydraulic leaks.
Traditionally, pressure transducers have been used to obtain approximate charge state data in accumulators. This approximation, however, is insufficiently accurate to be used in a regenerative system because of changes in ambient temperature. Due to these changes the pressure of a gas within an accumulator may vary significantly for a given volume. Additionally, by using pressure alone to determine charge state, one must assume that there are no leaks or changes in the gas charge.
The most direct method for determining the charge state of an accumulator is to measure the position of a piston or bladder inside the accumulator. A piston or bladder's position within an accumulator is directly proportional to the available energy in the accumulator. However, obtaining an accurate position of the piston or bladder within an accumulator can be a difficult task. Internal pressures within an accumulator can exceed 5000 psi and a visible detection method is not possible or is often very inaccurate or difficult to implement. Additionally, an accumulator contains hydraulic fluid and any sensor with direct contact must be able to operate within such conditions and provide adequate sealing capabilities.
Therefore it is advantageous for sensing technology used in hydraulic system to be highly accurate, easily integrated, operate under high pressure and not interfere with the sealing required to maintain the high hydraulic pressure in which it operates.
Sensors, such as a linear variable displacement transducer (LVDT), are inductive sensor used to measure relatively short displacements within such a hydraulic systems. A critical drawback to any LVDT is its relatively short measurement range. Such a sensor is not practical suited to directly measure a long linear translation within a hydraulic accumulator.
It is therefore desirable to have a hydraulic accumulator with a linear position sensor installed inside the pressure vessel to provide positional information for the moveable element inside of the accumulator, such as a piston or bladder, however other type of accumulators are also contemplated. Such positional information provides the accumulator charge condition data for use in hydraulic systems such as vehicular regenerative braking systems and generalized industrial accumulator systems. Access to such charge condition data allows for optimized control and operation of the hydraulic system. The sensor may include a high-pressure signal connector for conveying the electrical signals into and out of the pressure vessel. The sensor may be fixed to the case of the accumulator or mounted in the neck of the accumulator and has a connecting cable that is attached to the moving element inside the accumulator. The accumulator may be of several types, including a piston-type accumulator or a bladder-type accumulator. The sensor may be installed on either the gas side of the pressure vessel or on the oil side of the pressure vessel.